I can't find brushes in .blend files
Maybe the version 2.9.1 isn't compatible with the version of the add-on, btw, 2.9.1 is not a release yet, is an Alpha version, and from my experience, in the alpha version there is not everything inside Blender activated or working.
Solution (Maybe): Search for another version of the brushes add-on or try with 2.9 of blender and go backward into the versions until you get the right one (because from the link you posted, on the left side, the specifications and details, show that is compatible from 2.7X (but keep an add-on up to date in Alpha version is going to be almost impossible
Hope this helps